ABSTRACT This proposal builds on the scientific premise that even though female sex is a major disease modifier in pulmonary arterial hypertension (PAH), the effects of estrogens on the right ventricle (RV) and pulmonary artery (PA) in PAH are pleiotropic and incompletely understood. We posit that selectively activating estrogen receptor (ER)? signaling is a more precise approach of harnessing protective estrogenic effects in the RV-PA unit that maximizes benefit while avoiding detriment. The goal of this proposal is to identify novel and therapeutically targetable mechanisms by which ER? exerts protective effects on all three compartments of the RV-PA unit in PAH. We provide evidence that ER? exerts direct and indirect RV-protective effects by activating bone morphogenetic protein receptor 2 (BMPR2) signaling to upregulate apelin in the RV and distal PA, and by decreasing collagen accumulation in the proximal PA. We hypothesize that ER? improves RV-PA coupling in PAH by 1) up-regulating cardiomyocyte apelin, 2) reducing proximal PA collagen accumulation and cross-linking, and 3) preventing PA endothelial cell (PAEC) apoptosis. We propose the following aims: 1) To test whether ER? attenuates RV pro-apoptotic signaling and improves RV contractile function via BMPR2-dependent increases in apelin, 2) To demonstrate that ER? increases collagenase-mediated collagen degradation and decreases lysyl oxidase family-mediated collagen cross-linking in the proximal PA, and 3) To determine whether ER? decreases EC apoptosis in the distal PA through a BMPR2- and apelin-dependent mechanism. We generated novel ER? knockout rats to study the role of ER? in robust models of RV failure, thus avoiding pitfalls of prior studies of sex hormone signaling that were limited by lack of RV failure and by lack of interrogation of the entire RV-PA unit. These studies will be complemented by investigations in BMPR2-deficient rats and apelin-deficient mice in SA1&3, and Col1a1R/R mice resistant to collagenase-mediated collagen degradation in SA2. We will complement our in vivo studies with experiments in cardiomyocytes and PAECs isolated from rodents with PH as well as RV tissues and PAECs from PAH patients. The proposed studies are significant, since they will 1) identify ER? as a critical modulator of RV function and establish a novel and therapeutically targetable ER?-BMPR2-apelin axis in the RV, 2) establish collagenase-mediated collagen degradation and inhibition of lysyl oxidase family-mediated collagen cross-linking as functionally important mechanisms of ER? in the proximal PA and 3) establish up- regulation of BMPR2 and apelin as a novel mechanism of action of ER? in distal PAs. Our studies are innovative, since they will identify ER? as a novel mediator of enhanced RV-PA coupling. Technical innovation is provided by use of a newly generated ER? knockout rat model and a new highly selective ER? agonist. Upon completion of the proposed studies, we will have identified the ?net effects? of ER? activation in all three compartments of the RV-PA unit and established ER? as a potent mediator of direct and indirect RV-protective effects. This will allow for developing novel, non-hormonal therapies targeting both the RV and PA.